The present invention relates generally to cable television (CATV) systems and particularly concerns a technique for downloading selected information into the subscriber terminals of a CATV system.
A CATV system typically comprises a headend which transmits a plurality of television programming signals through a coaxial cable, each television programming signal being transmitted over a respective 6 MHz RF television channel. Each system subscriber is provided with a home terminal which functions as an interface between the signals transmitted over the coaxial cable and the subscriber's television receiver. In an addressable CATV system, the subscriber terminal is responsive to various encoded data signals transmitted by the headend for selectively authorizing viewing by the subscriber of selected channels or programs and not others. Such selective authorization is facilitated by scrambling the video content of the transmitted signals and using the subscriber terminal to selectively unscramble for viewing only those signals which the subscriber has been authorized to receive.
The RF channel configuration or format used in a CATV system may conform to any one of a number of different standards. These standards include the FCC broadcast format, the so-called HRC (harmonically related carriers) format and the so-called IRC (incrementally related carriers) format. In the FCC broadcast standard, all RF cable channels are spaced by integral multiples of 6 MHz from each other except channels 5 and 6 due to a 4 MHz gap between channels 4 and 5. Cable operators have found that certain types of distortions are reduced when all of the transmitted channels have picture carriers on frequencies that are integral multiples of 6 MHz. Thus, some CATV operators have adopted the HRC transmission format in which all of the RF cable channels are shifted in frequency with respect to the FCC standard to assume the desired channel spacing. Other operators have adopted the IRC transmission format wherein the desired channel spacing is realized by shifting the frenquencies of only channels 5 and 6; in particular, by shifting these channels downward in frequency so as to assume a contiguous relationship with channels 2-4.
In accordance with the foregoing, it will be appreciated that any given cable operator can employ any one of a plurality of different transmission formats. This creates a problem for the manufacture of the subscriber home terminals in that the channel tuning function of the terminal must correspond to the particular channel transmission format of the CATV system in which it is intended to be used. One solution to this problem is to preset or preprogram the tuning function of each terminal at the factory in accordance with the transmission format of the CATV system in which the terminal is to be used. Such factory preprogramming is undesirable for a number of reasons including cost and the possibility of errors. A more desirable solution to the problem is that of producing terminals at the factory which are operable in multiple tuning modes and subsequently programming the terminal for proper operation after it is installed in a CATV system by downloading an appropriate tuning mode command transmitted by the headend. While certainly representing a desirable solution to the problem, such has heretofore not been practical, particularly in CATV systems employing in-band data transmission techniques, since accurate data reception requires that the terminal be properly tuned to a transmitted channel. Such proper tuning cannot, of course, be assured since the tuning mode of the terminal as delivered from the factory may or may not correspond to the transmission format being used.